This invention relates to a demodulator having a cross polarization interference canceling function for canceling the interference between a main polarization and a cross polarization. More particularly, the invention relates to a demodulator capable of canceling cross polarization interference even if there is a phase difference between a demodulated signal on the side of main polarization and a demodulated signal (interference signal) on the side of cross polarization.
A method making joint use of orthogonally polarized waves in which the planes of polarization of two carrier waves having the same frequency are made orthogonal to each other to suppress interference and form two co-channels is advantageous in that radio frequencies can be utilized effectively. Such a method is employed in digital multiplexed wireless apparatus and in other transmission apparatus. A deviation in the planes of polarization occurs in such a transmission apparatus owing to distortion of propagation path caused by falling rain or some other phenomena and it results in that one channel is acted upon by interference from the other channel. Accordingly, the apparatus is provided with a cross polarization interference canceller (XPIC) which suppresses such interference on the receiving end.
FIG. 15 is a diagram illustrating an example of the construction of the receiving section in a transmission apparatus which employs orthogonally polarized waves. As shown in FIG. 15, a receiving antenna 101 is connected to the input of an orthogonal polarizer 102. One output (a V-polarized output) of the polarizer 102 is connected to the input of a QAM demodulating unit 104a via a frequency converter 103a. A first output of the QAM demodulating unit 104a provides a first demodulated signal (Qch, Ich) to a subsequent stage via a cross polarization interference canceller 105a. The other output (an H-polarized output) of the orthogonal polarizer 102 is connected to the input of a QAM demodulating unit 104b via a frequency converter 103b. A first output of the QAM demodulating unit 104b provides a second demodulated signal (Qch, Ich) to a subsequent stage via a cross polarization interference canceller 105b.
A second output of the QAM demodulator 104a enters the cross polarization interference canceller 105b and is used in order to cancel the interference between the cross polarized waves. The second output of the QAM demodulating unit 104b enters the cross polarization interference canceller 105a and is likewise used in order to cancel the interference between the cross polarized waves.
FIG. 16 is a block diagram of a demodulator provided on the side of main polarization and equipped with the cross polarization interference canceling function. Here the V-polarized waves are the main polarized waves and the H-polarized waves represent the cross polarization. The demodulating unit 104a is on the side of the V-polarized waves and so is the cross polarization interference canceller (XPIC) 105a The construction of the demodulator on the cross polarization output side has a similar construction.
An intermediate-frequency signal resulting from the frequency conversion performed by the frequency converter 103a is applied to one input of each of two mixer circuits 111a, 111b which construct a quadrature demodulator. A local oscillator 112 which oscillates at a carrier frequency fc applies its output to a hybrid device 113. The latter separates this signal into two signals that are 90.degree. out of phase and applies these signals to respective ones of the mixer circuits 111a, 111b at the other input terminals thereof. The output signal of the frequency converter 103a is orthogonally detected by being mixed with the two orthogonal signals in the mixer circuits 111a, 111b and is separated into a baseband in-phase signal and quadrature signal. The baseband in-phase signal (I-channel signal) and quadrature signal (Q-channel signal) have higher harmonics eliminated by low-pass filters 114a, 114b, respectively.
Next, using a sampling clock f output by a voltage-controlled oscillator (VCO) 115 of an internal PLL synchronized to the modulation frequency f of the received signal, A/D converters 116a, 116b convert the in-phase signal and quadrature signal to digital I- and Q-channel signals, respectively, each consisting of, e.g. eight bits. The digital data output by the A/D converters 116a, 116b is such that, in case of 16 QAM, the two higher order bits of eight bits represent digital information allocated to the I- and Q-channel signals. The six lower order bits serve as a signal which represents, in the form of a digital value, error produced by waveform distortion or the like.
The digital I- and Q-channel signals enter a transversal equalizer (TVEQ) 117, which subjects the signals to waveform equalization processing and eliminates transmission path distortion and quadrature distortion. More specifically, the transversal equalizer 117 (1) compensates for transmission path distortion components of the I- and Q-channel signals by digital signal equalization processing, (2) compensates for quadrature components by canceling the Q-channel signal component contained in the I-channel signal and the I-channel signal component contained in the Q-channel signal, and (3) enters the compensated I- and Q-channel signals into subtractors 121a and 121b, respectively, of the cross polarization interference canceller (XPIC) 105a.
FIG. 17 is a diagram showing the construction of a well-known two-dimensional transversal equalizer capable of being used as the transversal equalizer 117. The equalizer includes transversal filters 201a, 202a for eliminating transmission path distortion of the I-channel signal, transversal filters 201b, 202b for eliminating transmission path distortion of the Q-channel signal, and subtractors 203, 204. The subtractor 203 subtracts the Q-channel signal from the I-channel signal to cancel the quadrature component (Q-channel component) contained in the I-channel signal. The subtractor 204 subtracts the I-channel signal from the Q-channel signal to cancel the quadrature component (I-channel component) contained in the Q-channel signal.
Each of the transversal filters 201a.about.202b is constituted by an N-tap FIR filter (not shown) in which the coefficients can be changed. The filters decide coefficients so as to compensate for transmission path distortion. As mentioned above, when the I- and Q-channel signals are each expressed by eight bits in 16 QAM, the two higher order bits represent data and the six lower order bits represent the error due to waveform distortion, etc. The relationship between identification threshold values of two higher order bits and digital data is illustrated in FIG. 18. (1) When a third bit E is "1", the digital data is greater than an intermediate value (the dashed line) of the identification threshold values. (2) When E is "0", the digital data is less than the intermediate value.
In order to eliminate the effects of transmission path distortion, it will suffice to perform control in such a manner that the value of the six lower order bits become equal to the intermediate value (the ideal value) of the identification threshold values. The transversal filters 201a.about.202b eliminate the influence of transmission path distortion by causing the coefficients of the FIR digital filters to converge toward predetermined values in accordance with the above-described logic.
With reference again to FIG. 16, A/D converters 122a, 122b of the cross polarization interference canceller (XPIC) 105a use the sampling clock output by the voltage-controlled oscillator 115 to convert the I- and Q-channel signals that enter from the QAM demodulating unit 104b (see FIG. 15) on the side of the H-polarized waves to 8-bit digital signals. The resulting digital I- and Q-channel signals enter a transversal equalizer (TVEQ) 123, which applies waveform equalization processing, removes transmission path distortion and quadrature distortion and applies the resulting I- and Q-channel signals to the subtractors 121a, 121b, respectively. The subtractor 121a subtracts the I-channel signal (interference signal) of cross polarization (H-polarized wave) from the I-channel signal of main polarization (V-polarized wave) and outputs the difference, and the subtractor 121b subtracts the Q-channel signal (interference signal) of cross polarization (H-polarized wave) from the Q-channel signal of main polarization (V-polarized wave) and outputs the difference. In other words, even though part of cross polarization acts upon the main polarization as interference, the cross polarization interference canceller (XPIC) 105a eliminates the interference component from the main polarization, thereby making it possible to output the correct I- and Q-channel signals of main polarization.
A controller 124 in the XPIC 105a has a carrier wave reproducing circuit 124a for outputting a carrier-wave frequency control signal from the I-channel signal. A low-pass filter 118 smoothes this control signal and enters the smoothed signal into the local oscillator 112 as a control voltage. The local oscillator 112 controls the local oscillation frequency in accordance with the control voltage. The controller 124 further includes a clock synchronizing circuit 124b for outputting a clock-frequency control signal. A low-pass filter 119 smoothes this control signal and enters the smoothed signal into the voltage-controlled oscillator 115 as a control voltage. The voltage-controlled oscillator 115 controls the sampling clock frequency in accordance with the control voltage so as to synchronize the clock to a symbol clock (the clock of the input digital data).
The arrangement of FIG. 16 makes it possible to cancel transmission path distortion, quadrature distortion and interference between channels.
In order to cancel interference between channels correctly, it is desired that the phases of the I- and Q-channel signals of main polarization and the phase of the interference signal coincide. However, the phase of the demodulated signal of main polarization and the phase of the interference signal differ owing to signal delay time and multipath fading of the transmission paths. This disparity in the signal phases diminishes the performance with which the interference between channels is canceled. For this reason, a phase adjuster such as a phase shifter is provided between the voltage-controlled oscillator 115 and the A/D converters 122a, 122b on the side of the different polarized wave and the phase of the sampling clock output by the voltage-controlled oscillator 115 is adjusted in the phase adjuster by an amount commensurate with the phase difference between the demodulated signal on the side of the main polarized wave and the interference signal. FIG. 19 is a diagram useful in describing the circumstances set forth above. Here MW represents the demodulated signal of main polarization, DW the demodulated signal of cross polarization, which is the interference signal, and IW the interference signal component superposed upon the demodulated signal of main polarization. A delay time td exists between the cross polarization signal DW constituting the interference signal and the interference component IW superposed upon the signal MW of main polarization. Consequently, when the main polarized signal and the cross polarization signal (the interference signal) are A/D-converted using the same sampling clock Cs on the side of main polarization and on the side of cross polarization and the interference signal is subtracted from the main polarized signal, the interference component cannot be canceled correctly. Accordingly, a sampling clock Cs', which is obtained by delaying the phase of the sampling clock Cs by the delay time td is used as the sampling clock on the side of cross polarization.
FIG. 20 is a diagram showing the construction of a demodulator provided on the side of main polarization and equipped with a phase adjuster. Here the demodulator has a digital configuration.
As shown in FIG. 20, the demodulator includes a frequency converter 301 for converting the frequency of the main polarized signal to an intermediate-frequency signal, a local oscillator 302 for outputting a local oscillation frequency signal, an A/D converter 303 for A/D-converting the intermediate-frequency signal in sync with the sampling clock Cs, an A/D converter 304 for A/D-converting the intermediate-frequency signal of the cross polarization, which enters from the side of the cross polarization waves, in sync with the sampling clock Cs', a voltage-controlled oscillator (VCO) 305 for generating the sampling clock Cs, a phase adjuster 306 for shifting the phase of the sampling clock Cs, a digital quadrature demodulator 307 for multiplying the output of the A/D converter 303 by digital cos.omega.t, -sin.omega.t signals to thereby generate baseband I- and Q-channel signals on the side of main polarization, and a digital quadrature demodulator 308 for multiplying the output of the A/D converter 304 by digital cos.omega.t, -sin.omega.t signals to thereby generate baseband I- and Q-channel signals on the side of cross polarization. The digital cos.omega.t, -sin.omega.t signals can be generated by discretely storing values of cos.omega.t, -sin.omega.t in a ROM in advance and then successively reading the values of cos.omega.t, -sin.omega.t conforming to each moment of time out of the ROM.
The demodulator further includes roll-off filters 309, 310 constituted by FIR filters for imparting a roll-off characteristic to the main polarization I- and Q-channel signals output by the quadrature demodulator 307, roll-off filters 311, 312 constituted by FIR filters for imparting a roll-off characteristic to the cross polarization I- and Q-channel signals output by the quadrature demodulator 308, and transversal equalizers (TVEQ) 313, 314. The transversal equalizer 313 corrects transmission path distortion and quadrature error contained in the I- and Q-channel signals on the side of the main polarized waves, and the transversal equalizer 314 corrects transmission path distortion and quadrature error contained in the I- and Q-channel signals on the side of cross polarization. Further, a subtractor 315 subtracts the cross polarization I-channel signal component from the main polarization I-channel signal, and a subtractor 316 subtracts the cross polarization Q-channel signal component from the main polarization Q-channel signal. A controller 317 outputs a signal CRF for controlling carrier wave frequency and a signal CLF for controlling sampling clock frequency. A low-pass filter 318 smoothes the signal CRF and enters the smoothed signal into the local oscillator 302 as a control voltage to control the oscillation frequency of the local oscillator. A low-pass filter 319 smoothes the signal CLF and enters the smoothed signal into the voltage-controlled oscillator 305 as a control voltage to control the sampling clock frequency of the voltage-controlled oscillator.
The phase adjuster 306 performs an adjustment to optimize the phase of the interference signal. Though the phase difference between the demodulated signal on the side of the main polarized waves and the interference signal differs depending upon the signal delay time and the multipath fading of the transmission paths, the adjusted phase generally is fixed.
In recent years, however, a demodulator equipped with a cross polarization interference canceling function for automatically regulating the amount of adjusted phase has been investigated and proposed for the purpose of eliminating the need for cable adjustment at the time of equipment installation and for improving upon the amount of XPIC compensation at the time of fading.
FIG. 21 is a diagram showing the construction of a demodulator provided on the side of main polarization and equipped with a function for adjusting phase automatically. Elements identical with those shown in FIG. 20 are designated by like reference characters. The arrangement of FIG. 21 differs from that of FIG. 20 in that (1) the controller 317 is provided with a function which decides the phase of the sampling clock (2) the phase control signal PSC enters the phase adjuster 306 from the controller 317, and (3) the phase adjuster 306 controls the phase of the sampling clock Cs' of A/D converter 304 based upon the phase control signal PSC.
The arrangement of FIG. 21 makes it possible to adjust the phase of the sampling clock automatically. As a result, the phase of the interference waves (cross polarization) is adjusted automatically so as to be the same as the phase of the demodulated signal of main polarization and the interference component can be removed from the demodulated signal of main polarization.
With the conventional method of controlling the phase of the sampling clock described above, a new problem arises when the phase difference becomes large in size and clock phase is controlled over a wide range.
When the clock phase is controlled by the phase adjuster 306, the phase of the digital output signal from the A/D converter 304 naturally varies by following up such control, and the phases of all subsequent digital signals also change by following up the variation. However, the compensated outputs of the transversal equalizer 313 on the side of cross polarization are subtracted from the output signals of the transversal equalizer 313 on the side of main polarization in the subtractors 315 and 316. As a consequence, it is necessary to change over to the clock phase on the side of main polarization by the time the subtractors 315, 316 are reached. Changing over the clock is achieved by storing the data on the side of cross polarization in memory temporarily by the clock Cs' and reading the data out of the memory immediately thereafter by the clock Cs on the side of main polarization, followed by processing the data. If the phase difference is large, however, clock changeover becomes impossible. The result is a narrowing of the range of clock phase adjustment in the A/D converter on the interference side (the side cross polarization). This means that phase cannot be adjusted if the phase difference is too large.